Die to obtain pieces of plastic materials molded by injection, projection or blowing processes and manufacturing method of said die. There are also provided herein, the methods to manufacture or repair mentioned die, and a process to modify a silicon rubber by addition that will be useful for said processes.
The present invention describes a die to manufacture pieces of plastic materials, molded by injection, projection or blowing, that does not need end finishing during the construction nor the employment of demolding agents during its whole useful life. The above mentioned die is specially useful to manufacture pieces of plastic materials that need painting in their finishing, or to manufacture those pieces that need to be superficially treated for their use in various industries, such as the automobile industry, electro-domestic accessories manufacturing, communications, computers, or decoration, etc. This die is also specially designed to manufacture pieces that are to be efficiently and permanently bound to others, as are the soles for the industrial manufacturing of shoes and of sports shoes.
The invention also describes the methods to manufacture or to repair the above mentioned die, and the process to prepare a modified silicon rubber by addition, herein referred to as SRBA, that is modified for the purpose of obtaining greater utility for the mentioned processes.
The use of demolding agents to manufacture pieces of different plastic materials molded by injection, projection or blowing is known in the art, and those agents may be for internal or external use.
Internal demolding agents are blended in the bulk of the plastic material to be processed, so, the whole compound obtained during this process remains with demolding agent residues on its surface. This makes the article unsuitable to be painted or bound, even if it is submitted to a further complex or very expensive operation, as the demolding agent is also mixed in the bulk of the article.
On the other hand, the external demolding agents, commonly prepared in a silicon basis, have to be directly spread on the die before its use to prevent the pieces from binding with each other during the whole process. Within the pressure and temperature conditions existing during the injection, projection or blowing processes, part of the demolding agent applied to the die is inevitably transferred to the external surface of the article, with a thickness that can be measurable which, therefore, turns difficult the whole removal of the agent.
As a third possibility, we may also mention those materials that, as the polypropylene, do not require the use of demolding agents to be processed, as they naturally reject to be bound to other materials. However, the die and the manufacturing method described in present invention may be useful with these materials if the die of this class is of a less expensive cost than those traditionally used.
Nowadays, the plastic industry has to overcome the problems caused by the presence of the demolding agent, or by the material chosen as raw material, in order to achieve a good painting or a good binding of the injection, projection or blowing molded pieces of plastic materials.
It is not possible to paint or to bind the articles if they are manufactured using an internal demolding agent. The presence of the said demolding agent, or of the raw material, continues with its demolding action and does not allow a good adherence either of the paint or of the adhesive to the manufactured article.
If the pieces are manufactured with an external demolding agent, they must be submitted to a previous process in order to paint them. Those processes are very complex, expensive and contaminate the environment, as for example, the continuous washing with isopropyl alcohol, flamed process, granulated process, etc. These processes, either individually or altogether, have an ambiguous result, as the demolding agent cannot be wholly removed from the surface of the pieces to be painted or bound. That is to say, the residual demolding agent is always present in the manufactured article.
When the articles are manufactured without using a demolding agent, it must be taken into account that they also have to be submitted to various processes to adhere the paint or the adhesive to them. Therefore, pieces are flamed under a controlled flame step, to open the material pores. They are submitted to a crown process, that is to say, a bombardment of their surfaces applying high voltage to obtain the uprising of material microscopic filaments. Therefore, the surface exposed to contact is increased to produce a kind of texture that helps the adherence.
Patent EP 0294043 to Dow Corning, describes a method for molding high water content, low flexible stress, low flame-ability and good supporting load characteristics polyurethane foam articles, wherein a permanent demolding agent is used. A releasing external silicon compound for dies is used to treat the die surfaces. The compound conveys durability features in the die releasing process due to the multiple molding cycles, allowing to cover the die with said compound and so producing such free from defects foam articles. The compound essentially comprises polydimethyl siloxane in blocks ended with high or low molecular weight hydroxyl, a siloxane crosslinking agent having SiH functionality, a catalyst, and an inert solvent.
PCT WO 91-011208 to Robert William Wilson et al., describes an internal demolding agent which is absorbed in a micro-porous carrier, being said carrier afterwards incorporated into the base material, as, for example a plastic material to be molded. The demolding agent is scattered on the base material bulk, making easier the release of the molded component from the mold or die, without a previous spray of an additional demolding agent to said mold or die. Consequently, the molded articles cannot be painted or bound because the action of the demolding material is spread and incorporated in the bulk. The articles obtained by this process exhibit serious adherence difficulties in coating by means of the application of paints over their surfaces.
U.S. Pat. No. 3,838,949 describes a mold that does not need a demolding agent, wherein at least the surface that is in contact with the molding compound includes cured polyester. Said cured polyester contains fluorocarbons prepared from polyepoxy, having an epoxy functionality of about two epoxy groups per mole, and a polymer with a carboxy end. Said polymers with carboxy ends contain pending allyl or vinyl groups, and wherein at least some of said pending groups are converted into 2-perfluroalkyl-1-iodoethyl, or 3-perfluroalkyl-2-iodopropil groups, to provide a 1 to 40 percent of fluoride content.
U.S. Pat. No. 3,838,110 describes a compound with a mold releasing capacity that may be used as molding compound, or it describes a mold or molding coating in its cured state. It is not necessary to use an additional demolding agent when the compound described is used, which is prepared from (a) polymers ended in carboxy or hydroxy which contain 3-perfluroalkyl-2-iodopropyl or 2-perfluroalkyl-1-iodoethyl pending groups, and (b) polyisocyanate or polyepoxy compounds.
These last two patents have been the first steps taken in the process of manufacturing molds to be employed without the use of any demolding agent; but as they were expensive and they did not achieve good results, they were discontinued. Besides, the Teflon based material was often scratched and torn while, besides, the construction of the dies took a long time to be carried out.
The SRBA was developed and afterwards employed as a permanent demolding agent, but due to its strong resilience and the impossibility of firmly binding it to any stiff element, its use has always been limited to gravity strain molds, as these molds do not resist pressure without distortion.
This difficulty has been largely described and dealt in the art. Repeated attempts have been carried out to combine the SRBA with other materials that could supply the necessary resistance to distortion in a sufficient and necessary quantity to use SRBA in dies manufacture, but these attempts had always faced the difficulty that SRBA does not bind itself to materials used.
This invention has solved this problem, obtaining a proper SRBA thin layer adherence over metals, laminated plastics, or any other element that delivers sufficient mechanical resistance, making possible the use of a new system in the die for plastic injection, projection or blowing processes.
The present invention is related to a die construction technique that makes unnecessary the use of any kind of demolding agent during the plastic material processing, which are either of internal or external use. In fact, since the different processes of mentioned materials are known, it has always been necessary the use of agents to prevent the natural adherence to any element that entered into contact with the materials during the process.
This is the cause of a number of problems among which, we can mention, cost increase due to the necessity of adding a demolding agent, waste of time during the manufacture of pieces, failures or brakes during the release of the articles, together with the subsequent damage on dies or molds, and the impossibility of achieving a good painting and/or end finishing adherence of the processed pieces. Industries face similar difficulties when binding the plastic material articles obtained by means of a process of injection, projection or blowing molding, as for example, in the sport shoes manufacturing process.
Taking into account the difficulties that have been described before, and considering that the use of painted plastics is nowadays in constant increase in the industrial field, it is important to remark that during the injection, projection or blowing processes, or during any other similar method carried out to manufacture an article, the presence of demolding agents, under the usual temperature and pressure working conditions, results in the incorporation of said demolding agents into the manufactured pieces. Therefore, we can remark that the above mentioned residual demolding agent does not allow a good finishing adherence of the molded pieces. Consequently, in order to neutralize this difficulty, the industry carries out a variety of operations after the release and before the end finishing layer application, operations that are always very complex and make the product more expensive. In spite of all the measures that are taken in this sense, the quality of the piece end-finishing adherence is deficient. The attempts carried out and the poor results obtained in the automobile industry, regarding the car bodies, are a clear example of what has been mentioned before.
In the present invention, the SRBA continues acting as a demolding agent. But, as it is used as a very thin layer and bound to a stiff substratum that is really the one that keeps the forms, it allows to submit the die at a pressure, which contact surface with the material under process is the SRBA. The result is that the injection die obtained does not need the use of any demolding agent during its useful life.
The die of present invention differentiates from the injection dies that, at present, are manufactured in metal or synthetic resins, because the additional external or internal demolding agents have to be unfailingly used.
The surface of the article obtained, according to present invention, is similar to that of a metal submitted to cataphoresis by the intermolecular bond existing between the base product and the product applied when the first one is painted or bound, as if the paint or the adhesive could diffuse within the surface of the piece, differentiating from the end finishing of the pieces carried out by other processes wherein the paint or adhesive remain floating over the surface of the piece.
It is therefore a first object of this invention, to provide a die to obtain plastic pieces by injection, projection or blowing, not needing said piece final finishing during its manufacturing process nor the use of additional demolding agents during its whole useful life.
A second object of the invention is to provide a process to obtain a modified SRBA; being said modified SRBA useful to manufacture the referred to die.
It is a third object of the invention to provide a process to manufacture said die with said SRBA.
Besides, another object of this invention is to provide an alternative process to manufacture a die using said SRBA, wherein the manufacturing time is substantially shorter.
It is another object of this invention to provide a process to repair a die manufacture according to any of the processes described, taking out by mechanic abrasion or thermal process the two first layers to replace them afterwards employing the corresponding steps of any manufacturing process described above.